Tutorial 2: Debugging Anton Gerdelan Debugging Tools

• Pencil and paper (diagrams of …., manual step- through)

• printf(), assert()

• Text editor, terminal

• Eyes, brain

• program - gdb, lldb, the one in visual studio

• A good front-end is nice The Problem with

• are convenient for following the path code takes - functions, recursion, templates…

• Linux has a major dearth of visual debuggers

• gdb and lldb have a command line interface (okay for getting a backtrace after crash … clunky otherwise)

• DDD, , Code::Blocks, CGDB - none great “Alright I’ll Just Make My Own Then!”

“oh…that’s why there aren’t any decent ones…” Enter: VS Code

• is very nice (simple, quick, visual)

• type in “code” in terminal to open

• install+enable the “/C++ extension” (puzzle piece icon)

• you get a text editor

• , watch-list, stack trace, git integration,

• you don’t have advanced views; memory inspect, asm, etc. Setup

• File->”Open Folder” gives you a ‘project’ view

• Open or create a new main.c file

• first let us compile the program (you can use the built-in terminal or an external one)

• gcc -o my_demo main.c -g -std=c99 (Linux)

• -o my_demo main.c -g -std=c99 (OS X or Linux)

• mingw32-gcc my_demo main.c -g -std=c99 (Windows with mingw gcc)

• -g means “include debugging symbols in my program”

• you won’t be able to debug without them Start Debugging

• View->Debug

• Click in the margin (left of line numbers) to set a (red circle)

• the cog button takes you to settings file .vscode/ launch.json

• which debugger to use, enter your program name to run, etc. Stepping

• Green arrow starts program in debug mode

• Program pauses on breakpoint line

• Use step over (next line) step in (next line but also go into functions) step out (run until we leave the function) continue (green arrow) stop (red square)

• local variables’ values (in scope)

• hover mouse over a local variable

• right click variable to add to watch list

• call stack of functions open with line that call the next function Useful For

• Checking why variables or output do not have the value you expected - “what am I missing?”

• Check the flow of execution (esp. other peoples spaghetti code)

• Why lead up to it crashing?

• “binary search” your breakpoints around The command-line version

• After compiling with debug symbols

• gdb ./my_demo (or lldb ./my_demo for OS X)

• This enters a gdb terminal session

• type run

• after it crashes type bt to get a backtrace

• which line caused the and what leads up to that

• type q to quit gdb Alternatives

• On Apple install in Appstore to get all the programming tools (clang, lldb, etc.)

• Xcode and Visual Studio are pretty handy (but very bloaty with all the project settings)

• I still prefer gcc on Windows (but that’s me)

• Some people like Code::Blocks (in lab) and on Linux Other Useful Tools

• Profilers {gprof (linux), Instruments (Xcode), VTune}

• Static Analysis - very handy code mistake finder - try Clang’s static analysis tool on your code files.

• Memory leak checker - Valgrind

• Friends - code review / tips / sanity check